Pesticides used in farming are killing bumblebees and affecting their ability to forage, putting colonies at risk of failure, according to a new study.An estimated one-third of all plant-based foods eaten by humans rely on bees for pollination, and bees and other pollinators have been estimated to be worth around $200 billion a year to the global economy. However, bee numbers have been plummeting in recent years, particularly in North America and Europe.

ADVERTISEMENT

As bees provide around 80% of pollination by insects, it is vital to understand and deal with the causes of these declines. Recent studies have suggested that pesticides may play a role in bee declines, as pesticide exposure can cause changes in bee behaviour and reduce the production of queens in colonies. However, the overall effects at the colony level are less well understood. Bees are also exposed to a number of different pesticides while foraging, but their combined effects have rarely been investigated.

In the new study, published in Nature, scientists exposed colonies of bumblebees to the pesticides neonicotinoid and pyrethroid over four weeks, at levels similar to those found in the field.

Most previous studies have focused on honey bees, which are smaller than bumblebees but have much larger colonies, sometimes numbering tens of thousands of individuals. In contrast, bumblebees form colonies of just a few dozen individuals, potentially making them more vulnerable to impacts at the colony level.

The findings showed that long-term exposure to the two pesticides impaired the foraging behaviour of the bees and increased worker mortality, leading to significant reductions in colony success. The researchers also found that being exposed to a combination of more than one pesticide increased the likelihood of a colony failing.

"Effects at the individual level can have a major knock-on effect at the colony level. That’s the novelty of the study," said Richard Gill, the lead author of the study.

According to the researchers, the new findings emphasize the need for wider testing of pesticides. They endorsed the recommendations of the European Food Safety Authority (EFSA) that pesticides should be tested over longer periods and that new protocols should be developed to detect the cumulative effects of multiple chemicals on bees. There should also be separate assessments for different bee species.